Control circuit for electromagnetic devices



Jan. 14, 1941. v w. SAVILLE 2,228,405

CONTROL CIRCUIT FOR ELECTROMAGNETIC DEVICES Filed March 26, 1938 swab/Vs HNAL c/Rcun F/NDER Y HE TOK F .H -QFUW 1 T mam INVENTOR WILL/E SAV/LLE Patented Jan. 14, 1941 UNITED STATES PATENT OFFICE CONTROL CIRCUIT FOR ELECTRO- MAGNETIC DEVICES ware Application March 26, 1938, Serial No. 198,183 In Great Britain April 26, 1937 8 Claims.

The present invention relates to improvements in circuits for electromagnetically-operated devices such as relays, meters or switch magnets suitable for use in telephone or like systems and its main object is to provide a simple method of securing an increased margin of safety where such devices are arranged so that current flows therethrough without producing operation which I is only effected when the current flow is increased.

10 According to one feature of the invention in an arrangement for controlling the operation of electromagnetic devices, a device arranged to be initially traversed by current which is not sufficient to cause its operation is connected in series with a unit comprising a solid body composed of material the resistance of which is substantially the same for both directions of current flow but which decreases with increase of voltage so that when the voltage applied to the combination is increased the reduction in the resistance of the unit will cause the increase in current through the device to be more than proportionate to the increase in voltage and thus effect its reliable operation.

According to another feature of the invention in an arrangement for efiecting the operation of one or other of two relays normally connected in series across a source of current in response to the application of a suitable potential to their junction point, the circuit includes two units composed of material the resistance of which is substantially the same for both directions of current flow but which decreases with increase of voltage, the two units being connected on opposite sides of the point to which the control potential is applied whereby the operating margin is improved.

A further feature of the invention is that in an arrangement for controlling the operation of an electro-magnetic device, the device is connected in series with a unit composed of material the resistance of which is substantially the same for both directions of current flow but which decreases with increase of voltage and is operated by the application of a distinctive potential thereto in place of a potential short-circuiting the device, both potentials serving also to operate a further device which is traversed by current in series with the first device when both potentials are removed, both devices being then unable to operate owing to the presence of the unit of nonlinear resistance material.

The non-linear resistance material for forming the units preferably comprises silicon carbide mixed with carbon and/or tungsten or molybdenum which is agglomerated under pressure and subsequently baked, the resulting mass having a resistance current characteristic which is substantially hyperbolic. The resistance material may be made into various shapes and conveniently is pressed into the form of a disc or washer and the surface metal-sprayed at appropriate points in order to permit the necessary electrical connections to be made as described in my copending application Ser. No. 198,182, filed March 26, 1938.

The invention will be better understood from the following description of two methods of carrying it into effect which should be taken in conjunction with the accompanying drawings comprising Figs. 1 and 2. Fig. 1 shows a simple arrangement in which two relays are permanently connected in series with the source of current but are required to operate only when a special potential is connected to their junction point. Fig. 2 shows the application of the principle to so-calledl booster battery metering in a telephone system in which the meter takes the place of one of the relays of Fig. 1.

Referring first to the arrangement of Fig. 1, it will be seen that the two relays A and B are connected in series in a circuit which includes two units of non-linear resistance material NLRI and NLRZ. From the junction point of these resistances extends a control conductor which may be of considerable length and to which either earth or battery potential may be connected by means of the switches X and Y respectively. Under normal conditions the resistances of the units NLRI and NLRZ are so high that neither relay is able to operate.

If the switch X is closed however, the voltage across the unit NLRZ will be increased and its resistance will therefore be lowered and the two effects in combination produce reliable operation of relay B. There is clearly no tendency for relay A to operate as the voltage applied to it will be reduced. Similarly if the switch Y is closed instead of the switch X the potential across the unit NLRI will rise and its resistance will therefore fall and this coupled with the fact that a greater voltage is applied to the combination of relay A and unit NLRI will cause reliable operation of relay A. It will be understood of course that when conditions are restored to normal the resistances of the units will again increase and this increase is preferably sufficient to prevent relays A and B remaining operated in series.

In one practical arrangement of the type shown the normal current flowing is 3 milliamperes while when either of the control switches is closed the current through the relay concerned rises to about 17 milliamperes. This gives a very satisfactory margin between the operating and nonoperating conditions and ensures reliable operation of the relays over a long period without fine adjustment.

It will be appreciated that by arranging for a battery with its negative pole earthedto be applied to the junction point the operation of both relays A and B may be effected and with this method of operation also the presence of the unit NLRI at any rate improves the response.

It will be understood also that the invention is not limited to the use of two relays in series and that it may prove equally advantageous in a case where only one relay is concerned, and the voltage across it is varied in order to distinguish between the operated and non-operated condition.

Referring now to Fig. 2, this shows diagrammatically sufiicient of the metering circuit of an established automatic telephone connection to enable the invention to be understood. The conditions represented are those which exist when the connection has been fully set up and it Will be seen that the connection to the meter M associated wi l the subscribers line circuit extends over the finder switch F and by way of a unit of non-linear resistance material NLR to the holding lead which extends through the various group selectors to the final selector in which it is connected to earth. This is the condition throughout the connection except when metering is teltin place which may be efiected when the calling party replies and also if desired at predetermined intervals throughout the connection, cr alternatively, on release of the connection. When this occurs, the armature a of the controlling relay is operated and thereupon connects up booster battery in place of the normal earth. This increases the current through the various holding relays such as I-IB so that they have no tendency to release and also increases the potential applied across the unit NLR, thereby causing it to lower its resistance considerably. The current now flowing is suflicient to operate the meter and wh n the booster battery connection is removed the resistance of the unit NLR again rises so that the meter is caused to release.

In the arrangement shown one terminal of the meter is connected to the unearthed side of the exchange battery and it is assumed that the booster battery is of substantially the same voltage, conveniently of the order of -50 volts. With the arrangement according to the invention however, it is not essential that the booster battery voltage should reach this value as the use of the unit NLR will permit satisfactory operation with a comparatively small change in the voltage applied to the meter.

It is also possible to have the meter connected to earth instead of to battery and this ensures its release and permits reliable multi-metering since on the removal of the booster battery it is short-circuited. The effect of the unit NLR is then to ensure that the holding relays such as HB cannot remain operated in series with the meter as might otherwise be the case. With this arrangement, however, the voltage of the booster battery must be appreciably greater than that of the exchange battery and this requirement may make the arrangement less satisfactory in practice.

I claim:

1. In combination, a series circuit including two electrical responding devices, two non-linear resistance elements and a source of current, the current flowing in said circuit being of insuficient value to operate either of said devices, and means for applying a potential to a point of said circuit, only one or only the other of said devices thereupon operating depending upon the value of the potential applied to said point.

2. In combination, a circuit including two resistance elements connected in series with one another and with a source of current, and means for applying a potential to a point in said circuit, one of said elements effective to decrease in resistance responsive to the application of said potential thereby to control a portion of said circuit in one manner, the other of said elements effective to increase in resistance responsive to the application of said potential thereby to control another portion of said circuit in a different manner.

3. In combination, a circuit, a source of current for said circuit, said circuit including two parallel legs each containing an electrical responding device, one leg also containing a non-linear resistance element to prevent operation of the device in that leg by the current from said source, the device in the other leg being operated by current from said source, and means for increasing the flow of current in said circuit without affecting the device in said other leg, the resistance of said element effective to decrease responsive to said increase of the flow of current in said circuit thereby to permit the device in said one leg to operate.

4. In combination, a circuit containing a pair of electromagnetic devices and a non-linear resistance element having the same resistance characteristics for current flow in both directions, a source of current connected to said circuit, means for applying one potential to a point in said circuit to thereby operate one of said devices, and means for applying another potential to said point of said circuit to cause said element to decrease in resistance thereby to operate the other of said devices.

5. In combination, a circuit containing a pair of electromagnetic devices, a resistance element and a source of current connected in series, means normally effective to shunt one of the devices and said element thereby to operate the other device, and means for disconnecting said shunt and for connecting to said circuit a potential of suflicient value to maintain said other device operated, the resistance of said element decreasing responsive to the connection of said potential to said circuit, where upon said potential is effective to operate said one device through said element.

6. In a telephone system wherein metering of calls is effected over a holding conductor which has exchange battery normally connected thereto during a call, a non-linear resistance element, a meter connected to said conductor through said element, said element thereby preventing the operation of said meter by said exchange battery, a booster battery, and means for connecting said booster battery to said conductor to decrease the resistance of said element and thereby operate said meter.

7. In a telephone system, a normally grounded control conductor, one or more relays operated from the ground on said conductor, a non-linear resistance, a meter connected to said conductor through said resistance, said resistance preventing said meter from operating from the ground on said conductor, a booster battery, and means for disconnecting the ground from said conductor and for connecting thereto said booster battery, said booster battery effective to maintain said one or more relays operated and to operate said meter over said resistance, said resistance decreasing in value responsive to the connection of said booster battery to said conductor.

,8. In a control arrangement, a circuit having two branches, an electromagnetic device in each branch, a non-linear resistance element in one of said branches, a source of current connected to said circuit and normally causing an initial flow of current in said devices, means for connecting one potential to a point in said circuit between said branches to cause the operation of one of said devices, and for connecting another potental to said point to cause a decrease in the resistance of said element and the consequent operation of said other device.

WILLIE SAVILLE. 

